Parathyroid function in different stages of vitamin D deficiency rickets

Direct measurements of parathyroid activity are available in only small numbers of children with vitamin D deficiency rickets (VDR). Therefore serum immunoreactive parathyroid hormone (iPTH) and the urinary cyclic adenosine-3',5'-monophosphate excretion (UcAMP) were measured together with other important indices of calcium metabolism in 24 patients (aged 2-42 months) with VDR before vitamin D treatment. iPTH and UcAMP were significantly elevated in comparison to age-matched controls. In patients there was a highly significant positive correlation between iPTH and UcAMP and a negative relationship between both indices of parathyroid activity to serum phosphate and urine calcium, respectively, indicating that the simple measurement of serum phosphate and/or urine cAMP and Ca provides a reliable tool for the assessment of secondary hyperparathyroidism in VDR. In two patients classified as being in the early stage of VDR the parathyroid activity was not elevated despite hypocalcemia indicating relative hypoparathyroidism. Twelve patients with VDR were followed during vitamin D therapy: Within the first 2 weeks of treatment UcAMP slightly increased and thereafter decreased in most patients, but was still elevated in three patients even after 7 weeks, whereas iPTH became normal within 3 weeks of treatment. This favors the concept that vitamin D deficiency diminishes the activation of renal adenylate cyclase by PTH which is overcome by the highly increased PTH secretion in the advanced stages of rickets. The basal and calcium-stimulated serum calcitonin (CT) levels, determined in some of the patients, were normal, ruling out a significant disturbance of CT secretion in VDR.